Here is a partial solution I have found to the file "corruption" problem in the G-70:
let's say that, after organizing your database you add files to a folder (like "Put new styles here") using your computer via USB, then you disconnect the G-70 and, to your frustration, discover that it does not recognize the styles you have just added... then all you have to do is connect the computer again, go to the "DB" (database) folder and erase all the files relating to the styles (they have the suffix "sty", so are easy to recognize).
When you are done, disconnect the G-70 again, press the Styles Music Finder button, choose the memory (internal or external) and watch the G-70 do the organizing job again... this time all of your styles should be there!
I agree that from the point of view of a Yamaha user all this is frustrating (why can't the G-70 ackowledge all the folder/files the way a Tyros 2 or even a cheap computer does?) , but once you've learned to communicate with it using its own language, things start to make sense.